查询每一组数据的第二条记录
select A.*
from A csdINNER JOIN (select max(id) as maxId, A from A t1where id < (select max(id) from B t2 where t1.c = t2.c) group by c) csd2 on csd.id = csd2.maxId and csd.c = csd2.c
where
csd.c = 421